The excitation voltage for a three-phase synchronous generator, such as used in an integral drive generator (IDG) for aeronautical applications, is controlled by turning on and off a solid-state switch using a Pulse Width Modulated (PWM) signal. The frequency of the PWM signal is kept low, usually about 3 times the Point Of Regulation (POR) frequency, to minimise losses in the switch. However, if the PWM frequency is not synchronised with respect to the POR frequency, significant modulation of the POR voltage occurs. One of the POR phase voltages commonly serves as a synchronisation source to prevent this occurrence. However, unbalanced load conditions, short circuit conditions, and open phase voltage sense conditions compromise this sort of synchronization source, thereby causing significant modulation of the POR voltage.